Survivors fled to higher ground on the South Pacific islands after the magnitude 8.0 quake struck at 6:48 a.m. local time (1:48 p.m. EDT; 1748 GMT) Tuesday.
Four tsunami waves 15 to 20 feet (4 to 6 meters) high roared ashore on American Samoa about 15 minutes after the quake, reaching up to a mile (1.5 kilometers) inland, Mike Reynolds, superintendent of the National Park of American /*Samoa*/, was quoted as saying by a parks service spokeswoman.
Military transports carrying medical personnel, food, water, medicines and other supplies were headed to the stricken islands.
"Right now, we're focused on bringing in the assistance for people that have been injured, and for the immediate needs of the tens of thousands of survivors down there," Federal Emergency Management Agency Administrator Craig Fugate said.
A Coast Guard C-130 plane loaded with aid and carrying FEMA officials was headed from Hawaii to American Samoa's capital of Pago Pago, where debris had been cleared from runways to allow for emergency planes to land.
Some islanders had enough warning to flee the tsunami. But a warning system run by the Global Security and Crisis Management Unit failed to evaluate the tsunami's impact in real time because of a hardware failure.
New Zealand's acting Prime Minister Bill English said tents, stretchers, the temporary morgue and a body identification team were sent to Samoa on a Hercules transport plane after a "specific request" from local officials, who are "are very concerned about the growing death toll."
The quake was centered about 120 miles south of the islands of Samoa, which has about 220,000 people, and American Samoa, a U.S. territory of 65,000.
Another strong underwater earthquake rocked western Indonesia on Wednesday, less than 24 hours after the Samoan quake, briefly triggering a tsunami alert for countries along the Indian Ocean. The 7.6-magnitude quake toppled buildings, cut power and triggered a landslide on Sumatra island, and at least 75 people were reported killed. Experts said the seismic events were not related.
The Samoan capital, Apia, was virtually deserted by afternoon, with schools and businesses closed. Hours after the waves struck, sirens rang out with another tsunami alert and panicked residents headed for higher ground again, although there was no indication of a new quake.
In Pago Pago, the streets and fields were filled with debris, mud, overturned cars and several boats as a massive cleanup effort stretched into the night. Several buildings in the city - just a few feet above sea level - were flattened. Power was expected to be out in some areas for up to a month.
Water service has been restored to many villages, but power is still out in most areas. More than 1,000 people spent the night in 15 emergency shelters.
President Barack Obama declared a major disaster for American Samoa, adding that he and his wife, Michelle, "will keep those who have lost so much in our thoughts and prayers."
Hampered by power and communications outages, officials in the South Pacific islands struggled to determine casualties and damage.
Hundreds of people bombarded American Samoa's radio stations with requests to announce the names of their missing loved ones. Broadcasters urged listeners to contact their families immediately.
Samoa National Disaster Management committee member Filomina Nelson told New Zealand's National Radio the number of dead in her country had reached 83 - mostly elderly and young children. At least 30 people were killed on American Samoa, Gov. Togiola Tulafono said.
The overall death toll was expected to rise as more stricken areas are searched.
Authorities in Tonga, southwest of the Samoas, confirmed at least six dead and four missing, according to English.
"I don't think anybody is going to be spared in this disaster," said Tulafono, who was in Hawaii for a conference. He added that a member of his extended family was among the dead. Joey Cummings of radio station 93KHJ in Pago Pago told the BBC that he watched from a balcony as a 15-foot tsunami wave struck, and "the air was filled with screams."
He yelled for people to run uphill, "but they just ran down the street away from the wave rather than make a sharp left and up the steep mountain just feet away."
A "river of mud" carried trees, cars, buses and boats past his building, which is practically at sea level, Cummings told the BBC. Some people searched for trapped survivors, he said, but others looted stores. Bodies were stacked in the back of pickup trucks, he added.
Alex Godinet, chief of staff for American Samoa's congressional delegate, said his "whole house and everything was shaking." When he went to the nearby village of Leone, the tsunami wave had already struck and receded.
"People, elders were trying to crawl all over the place, crawl up to higher place, higher areas," he told NBC's "Today" show. American Samoa's dominant industry - tuna canning - was affected. Chicken of the Sea's packing plant was forced to close, although the facility wasn't damaged, the San Diego-based company said.
New Zealander Graeme Ansell said the Samoan beach village of Sau Sau Beach Fale was leveled.
"It was very quick. The whole village has been wiped out," Ansell told New Zealand's National Radio from a hill near Apia. "There's not a building standing. We've all clambered up hills, and one of our party has a broken leg."
A New Zealand P3 Orion maritime surveillance airplane had searched for survivors off the coast of Samoa.
In Carson, Calif., High Chief Loa Pele Faletogo, president of the Samoan Federation of America, comforted Samoans in the U.S. who came to him seeking news of their relatives. The chief said he learned the body of one of his cousins, in her 60s, was found floating along the shore.
"I am the one people run to when things happen," Faletogo said. "One of the qualities of a chief is to calm people down even though it is hurting inside."
All 65 employees at the National Park of American Samoa were accounted for, said Holly Bundock, spokeswoman for the National Park Service's Pacific West Region in Oakland, Calif. The park service has 13 permanent workers and between 30 and 50 volunteers, depending on the time of year.
The Australian Department of Foreign Affairs said three Australians were among the dead. The British Foreign Office said one Briton was missing and presumed dead.
"So much has gone. So many people are gone," said a visibly shaken Samoan Prime Minister Tuilaepa Sailele Malielegaoi as he flew from New Zealand to Apia. "I'm so shocked, so saddened by all the loss."
He said his village of Lepa was destroyed. Although the alarm sounded on the radio and gave people time to get to higher ground, "not everyone escaped," he added.
Before boarding the C-130 with the FEMA officials in Hawaii, Tulafono said "each and every family" in American Samoa will know one of the dead.
The Samoa Red Cross estimated 15,000 people were affected by the tsunami.
Residents of both Samoa and American Samoa said they were shaken awake by Tuesday's quake for two to three minutes. It was centered about 20 miles (30 kilometers) below the ocean floor and was followed by at least three aftershocks of at least 5.6 magnitude. The effects of the tsunami could be felt nearly 5,000 miles away (7,500 kilometers) on a Japanese island, though there were no reports of damage or injuries there.
While the earthquake and tsunami were big, they were not on the same scale of the 2004 Indian Ocean tsunami, said Brian Atwater of the U.S. Geological Survey in Seattle. That tsunami killed more than 230,000 in a dozen countries across Asia.
Although the quakes in the Samoas and Indonesia struck within 24 hours of each other, experts said there was no link between them. "When you look at that, it's like, 'Oh something's going on there.' But researchers are convinced that because quakes are essentially a random process that they're not related," said Don Blakeman, an analyst for the U.S.-based National Earthquake Information Center.
Various factors explain why the Samoa earthquake caused a massive tsunami and the Indonesia quake, with a magnitude of 7.6, did not.
The difference in magnitude was one factor, Blakeman said. "It also has to do with the depth of earthquakes. The Samoan one was very shallow. The Sumatran one, I think, was about 80 kilometers (49 miles)."
Sagapolutele reported from Pago Pago, American Samoa. Associated Press writers Ray Lilley in Wellington, New Zealand; Jaymes Song, Mark Niesse, Herbert A. Sample in Honolulu, Cara Anna in Bangkok and Seth Borenstein and Michele Salcedo in Washington contributed to this report.